A particular feature of the amorphous metal alloys, also called metallic glasses, is that they do not have long-range atomic order. They are of considerable interest for mechanical applications as they can have a high breaking stress and a wide range of elastic loading. In general, metallic glasses have a far higher breaking stress than crystalline alloys with equivalent Young's modulus.
These materials have a very high Ashby index σ2/E, which positions them as materials of choice for making springs for energy storage. However, a study of the mechanical properties of metallic glasses shows that only metallic glasses based on Fe or Co would be capable of competing with the best known spring steels and alloys. Among these alloys, there are the Fe—Si or Fe—Co—Si or Fe—Si—B alloys used for their magnetic properties in the form of ribbons about thirty microns in thickness in the cores of inductors, as well as alloys intended for forming bulk metallic glasses, for example in [Gu et al., Mechanical properties of iron-based bulk metallic glasses, J. Mater. Res. 22, 258 (2007)]. It is also known that these alloys are brittle, either after shaping in the case of magnetic tapes, or intrinsically brittle in the case of bulk metallic glasses.
Now, mechanical application in clock and watch making, notably as a spring, requires tolerance to plastic deformation and/or fatigue strength, which implies a certain ductility of the material. Moreover, most of these alloys are magnetizable, which can cause disturbances of certain elements of a clock or watch movement, such as the oscillator.
Some scientific works mention the existence of plasticity for certain compositions of metallic glasses based on Fe or Co, for example Fe59Cr6Mo14C15B6 disclosed in the work mentioned above.
European patent application EP 0018096 relates to powders consisting of ultrafine grains of transition metal alloy containing boron, notably at the rate of 5 to 12 at %. These powders are intended for the manufacture of cutting tools.
European patent application EP 0072893 relates to metallic glasses essentially consisting of 66 to 82 at % of iron, of which 1 to 8% can optionally be replaced with at least one element selected from nickel, cobalt and mixtures thereof, from 1 to 6 at % of at least one element selected from chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium and hafnium and from 17 to 28 at % of boron of which 0.5 to 6% can optionally replaced with silicon and 2% at most can be replaced with carbon. These metallic glasses are intended for tape recorder reading heads, cores of relays, transformers and similar equipment.
International patent application WO 2010/000081 describes the use of a ribbon consisting of an amorphous metal alloy of formula Ni53Nb20Zr8Ti10Co6Cu3 as barrel spring.
The Japanese patent application published under No. JP 4124246 relates to a dial, which is a watch-making or clock-making component without any mechanical function. This dial need not display ductility or high elastic resistance, in contrast to a component such as a barrel spring. Moreover, the amorphous alloy is not used as such, but is crystallized before use. The alloy contains Zr and/or Hf as a requirement, as well as Fe and B, and the examples refer to an FeZrCuB alloy.
The Japanese patent application published under No. JP 57108237 describes an amorphous alloy for a clock or watch spring, which is not, however, a high-performance spring like a barrel spring. A requirement of the alloy claimed is that it should contain Si, P or C. The description mentions the use of B but no information is given about the quantitative compositions, and addition of Ni or Fe is not mentioned. Finally, the examples relate to alloys comprising Cr and P.
The European patent application published under No. EP 0942337 relates to a watch-making or clock-making spring consisting of an amorphous metal such as Ni—Si—B, Ni—Si—Cr, Ni—B—Cr and Co—Fe—Cr.
Despite numerous tests on compositions known in the state of the art, for example Fe59Cr6Mo14C15B6, the inventors were unable to obtain results usable for the intended applications in horology, owing to the brittleness of the material obtained in the form of ribbon. Therefore they searched for alloys specifically suited to the requirements of mechanical applications in horology.
For it to be able to be used in horology, an alloy must possess suitable mechanical properties (notably a very high breaking stress) and it must be possible for it to be cast or worked in the form of ribbon and to be shaped according to a very precise shape in order to maximize the energy stored by the spring.
More precisely, the inventors have defined specifications that an essentially amorphous metal alloy must satisfy in order to be used in a mechanical application in the field of horology, more particularly as a spring element, for example a simple spring such as a leaf spring, or an element obtained by cutting or stamping from a ribbon, or an element obtained by hot forming of a ribbon and/or by cold plastic deformation. Thus, the metal alloy must:                allow the production of a metallic glass (amorphous alloy) with a thickness of 1 micron or more, in the form of ribbon produced for example by rapid solidification (“melt-spinning” or “planar flow casting”), or in the form of thin wire produced for example by water quenching (A. O. Olofinjana et al., J. of Materials Processing Tech. Vol. 155-156 (2004) pp. 1344-1349) or by disk quenching (T. Zhang and A. Inoue, Mater. Trans. JIM, Vol. 41 (2000) pp. 1463-1466);        have high mechanical strength, preferably above 2400 MPa, or even above 3000 MPa.        
For a mainspring or barrel spring, the metal alloy must moreover:                be ductile in the form of a ribbon or wire as described above, i.e. does not break when stressed to 180° (diameter at break less than 1 mm when the ribbon or wire is folded on itself) and having a range of plastic deformation; and        preferably have a capacity for annealing, i.e. preserve its intrinsic ductility and its mechanical properties after heat treatment for forming.        
For a simple spring such as a leaf spring or for an element obtained by cutting or stamping from a ribbon, ductility and capacity for annealing are not essential. For a mainspring or barrel spring, ductility is essential and capacity for annealing is desirable to permit forming of the spring.
Moreover, it would be beneficial for the amorphous metal alloy to be paramagnetic in order to minimize the disturbances of the clock or watch movement in which it is integrated.